Marine fouling due to pernicious and pestiferous sessile organisms is a problem which reaches from ancient times to the present. One of the first successful anti-fouling compositions put into general use was patented in England in 1854 and comprised a hot mixture of a toxicant, copper sulfate, in yellow soap applied over a primer of rosin varnish and iron powder. Cuprous oxide was first used in a paint for ship's bottoms around 1863.
The most frequently specified toxicants or anti-foulants in early and recent patents are copper, tin, arsenic and mercury and their various compounds. Other materials have been mentioned such as strychnine, atropine, oxides of zinc, lead and antimony, creosote, phenol, metallic silver, iodine, bromine and mixtures of iron, copper and zinc powders.
Of the many patents which have been issued on paints and systems claiming to control fouling, only a few paints have been found effective for limited periods. Of the many anti-fouling compounds suggested for paints and coatings only three groups, namely cuprous compounds, tin and mercury compounds, have been useful, even in limited degree.
The most widely used poison for present day anti-fouling paints is cuprous oxide (CU.sub.2 O), while it must be noted that cupric oxide (CUO) is useless as an anti-fouling agent. Thus in utilizing cuprous oxide it is necessary to add an anti-oxidizing agent to the paint to prevent atmospheric oxidation to the inactive cupric form. Mercury compounds have also been used to a limited extent although they do not appear to be as effective as copper and are toxic to humans. Recently some tin compounds, notably tributyl tin oxide (TBTO) have been introduced and claimed to be highly effective (some 3 times more effective than copper). However, the cost is more than 10 times the cuprous compounds and the effectiveness drops off rather abruptly as opposed to the tapering effect of copper. Other inorganic compounds such as arsenious oxide, zinc oxide and nickel compounds have proven of little value though recent work with zinc oxide indicates that such is capable of reinforcing the longevity of life effectiveness of copper as an anti-foulant.
Prevention of fouling by use of toxic paint involves maintaining a lethal concentration of poison in the water immediately adjacent to the surface to be protected. There are two major problems which result from such an approach. Firstly, the leaching action of the poison will eventually exhaust the supply and the anti-foulant coating will no longer have any effect. Secondly, these poisons are toxic to humans and are a major source of pollution in busy harbors and in well traveled waterways. The ambient concentration of poison in some harbors is sufficient to prevent all plant growth in the area of the protected hulls.
One solution to the problems of anti-foulant coatings is the development of foulant release coatings, that is, coatings which do not allow the sessile organisms to firmly adhere to the boat hull. The action of foulant release can be combined anti-foulant poisoning action to permit the use of a more slowly leaching toxic agent.
U.S. Pat. No. 3,702,778, discloses the use of silicone rubber as an anti-fouling coating. Peroxide cure rubbers are demonstrated to reduce observable foul with time.
U.S. Pat. No. 4,080,190, discloses tin silicates employed as a binder or as a filler for anti-fouling coatings. The tin silicate can be cured as the coating or pre-cured, comminuted and used as a filler for organic coatings.
It is clear that silicone coatings which have a release action to foul may also have problems with adhesion to the substrate. This is true with prior art silicone anti-foul coatings.
Thus, it is an object of the present invention to produce a silicone anti-fouling coating which has both good release to foul and good adhesion to the substrate.